To what extent is it permissible to marry a girl from a different religious and social background, who has entered the path of guidance—partially—through the young man, given concerns about her being negatively affected by his conservative lifestyle, and anxiety over the future of any children?

"There is nothing like marriage for those who love one another." If the girl is committed to her religion, observes her prayers and hijab, then the difference in age or social class does not prevent marrying her. However, if you fear that these differences will affect your life together, then seek a woman who is suitable for you. Seek guidance from Allah (Istikhara) and entrust your affairs to Him. If your marriage to her is not facilitated, then cut off your relationship with her. It is permissible for you to assign some righteous Muslim women to her, so they may be a support for her in maintaining righteousness.